A matched filter circuit is a filter for judging the correspondence of two signals. In spread spectrum communications, a unique spreading code sequence is allocated to each user. The user finds a signal transmitted for that user by using a matched filter to which the user's own spreading code sequence is applied. When the signal received is coded for the user, the matched filter outputs a correlation peak for acquisition and holding.
Assuming a spreading code to be PN(i), a chip time to be Tc, a spreading ratio to be M, a time to be (t), an input signal at a time t to be S(t), and a correlational output signal at a time t to be R(t), the formula (1) can be obtained. EQU R(t)=.SIGMA..sub.i=0.sup.M-1 PN(i).multidot.S(t-i.multidot.Tc)(1)
Here, PN(i) is a 1 bit data sequence for each of the M possible values of i.
As a double or higher order of sampling is necessary for the acquisition, the calculation of the formula (1) is performed in a plurality of systems at the same time using a plurality of matched filter circuits, and the various calculation results are added. In order to realize such a matched filter, a digital circuit or a SAW (Surface Acoustic Wave) device was conventionally used. However, in using a digital circuit, the electric power consumption was large because the size of the circuit was large, and therefore it was not appropriate for mobile communications. In using a SAW element, it was not easy to realize a whole circuit by an element and the S/N ratio was low.
Since the spreading code for each value of i is a 1 bit string, the inventors proposed a matched filter, in the Japanese patent application No. 7-212438, that yields high-speed processing in a small-size and low power consumption LSI configuration by the following method: i) sampling and holding input signals as analog signals of time series, ii) branching out these signals into the signal groups "1" and "-1" by a multiplexer, and iii) adding the signals in each group in parallel by a capacitive coupling.
However, there was no clear suggestion in this application with respect to double-sampling.